It is already known to subject an advancing flat material to the action of an electro-magnetic radiation by passing it between the plates of a capacitor, for example to dry it or heat it. However, the use of such applicators raises difficulties; in particular, it is difficult to obtain a good uniformity of the treatment in the transverse direction of the material; furthermore, regulation of the treatment is very delicate, especially when the characteristics of the material vary as a function of the temperature of the material.
An applicator of this type is supplied by a generator of which the operational parameters are imposed by the manufacturer. To obtain sufficient conditions of stability, it is always necessary to dispose between the generator supplying the electro-magnetic energy and the applicator, an additional electro-magnetic circuit, commonly called matching box. Such matching boxes are most often constituted by inductors and capacitors and do not comprise resistive elements which are capable of dissipating heat and consequently reduce the total energetic yield of the installation.
However, Applicants have noted that, despite the presence of a matching box intended to regulate matching of the impedances of the generator and of the applicator, a phenomenon of thermal racing might be produced. This phenomenon is produced in particular when the dielectric constant of the material to be treated presents a considerable variation as a function of the temperature, even when the intensity of the electric field applied to said material is constant.
It has already been sought to avoid this phenomenon of thermal racing by measuring the temperature of the product and reducing the electric field applied. However, this solution is difficult to implement due to the inertia of the servo-mechanisms which are necessary.
Applicants' purpose is to propose an applicator which overcomes all the drawbacks observed, in that it makes it possible to obtain uniformity of the treatment in the transverse direction of the material and in that it is self-regulating, i.e. it corrects by itself the conditions of its functioning due to the variations of the characteristics of the material to be treated.
U.S. Pat. No. 3,532,848 already proposes a resonant high-frequency or micro-wave applicator for the treatment of a flat material whose functioning is independent of its width. This applicator comprises, in known manner, a flat capacitor between the plates of which the material to be treated moves. It comprises, as characteristic, on one side of the material, two successive plates connected together by a plurality of inductors or by a single inductor, continuous over the whole of their width.
The inductor constitutes with the capacitance of the flat capacitor an oscillating circuit whose functioning is independent of the width of the applicator. In fact, it is known that the value of the air inductor or self-inductance of a coil with contiguous turns is given by the formula: ##EQU1## in which R is the radius of the winding, l the length of the coil and .mu..sub.o the permeability of the vacuum. It is also known that the capacitance of a flat capacitor of which the plates are rectangular is given by the formula: ##EQU2## in which .epsilon..sub.o is the permettivity of the vacuum, l and d respectively the width and length of a plate and e the distance between the two plates.
In the present case, the inductor and the capacitance are connected in distributed manner over the whole length l of the inductor, depending on the width of the same dimension l of the capacitor.
The square of the resonance frequency of the oscillating circuit is equal to the reciprocal of the product LC. ##EQU3## By calculation, it is deduced that: ##EQU4##
It is therefore ascertained that the resonance frequency of the applicator is, in this type of applicator, independent of the width l of the plates.
However, an applicator as described in U.S. Pat. No. 3,532,848 is not self-regulating.
An applicator has now been found, and this is what forms the subject matter of the present invention, which makes it possible to obtain uniformity of the treatment in the transverse direction of the material and which is self-regulating.